The U.S. government is extending a program that allows borrowers with mortgages backed by Fannie Mae or Freddie Mac to refinance at lower rates.

The Home Affordable Refinance Program, known as HARP, was to expire Dec. 31. The Federal Housing Finance Agency, which oversees Fannie and Freddie, said Thursday it is extending the program through 2015 to enable more borrowers to use it. About 2.2 million people have refinanced their mortgages through the program since April 2009. Officials had hoped that at least 4 million borrowers would participate.

For borrowers to qualify, the amount owed on their mortgage must be more than 80 percent of the home’s appraised value. Borrowers must be current on their mortgage, which must have been sold to Fannie or Freddie before June 2009.